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154 Cards in this Set

  • Front
  • Back
How many AS numbers and who assigns?
65,535 and IANA assigns. Not enough for everyone to have there own AS.
Purpose of an Exterior Gateway Protocol?
To exchange routes between AS's.
Primary Reasons for BGP?
(1) Scaling a large network with 10's of thousands of routes. Internet has 275,000 routes and IGPs like OSPF and EIGRP cannot handles those.(2) BGP gives much more granular control because it has so many attributes.
Describe the three steps of BGP sharing routes (hint: RIB).
1. Receive update from neighbor (RIB–IN)2. Select best path based on policy routing (Local–RIB)3. Advertise best route to neighbor (RIB–OUT)
Does BGP support more than one address family?
Yes. IPv4, IPv6, and IPv6 multicast
What are the three BGP processes that are started when BGP is enabled?
BGP Router – responsible for calculating best path and interacting with BGP RIB.BGP I/O – handles processing of BGP control packetsBGP Scanner – walks the BGP rable and confirms the reachability of next–hops
Describes BGP's relationship with IGP as it relates to next–hop.
BGP routes point to next–hop address and it assumes the IGP is able to find the next–hop address. IGP routes usually point out interface and neighbor on interface to route toward.
As it relates to next–hop in the update, how is BGP different from any IGP?
IGP advertises themselves as the next hop and BGP advertises the actual next hop. Except for EBGP neighbor relationship.
Title BGP Peering
Title BGP Peering
What is the difference between IGP vs EGP as it relates to forming peers?
IGP leverages automatic discovery. They generally trust IGP routers, all routes go to all IGP routers. BGP must configure peers. Do not share details about network ie.bandwidth, delay. Just the path information.
What port does BGP use?
TCP port 179. TCP for reliability and sequencing. OSPF and EIGRP has it built in.
Why do IBGP neighbors only share routes one hop deep?
It is loop avoidance mechanism. BGP uses AS path for loop prevention. Within an AS the path never changes.
How many AS can a BGP router be in? How to configure it?
One.
If an AS is misconfigured can it be discovered?
Yes. The sending routers AS is reported via a debug message. It is in hex.
What is the administrative distance of external BGP routes, internal BGP routes, and local bgp routes? Why are they those values. Change please.
External BGP routes are 20 and internal and locally generated BGP routes are 200. AS Path prevent BGP distance of 20 from taking precedence for local routes learned through ebgp. 200 is higher than any IGP so it takes precedence over other
What does BGP assume about external BGP neighbors (hint: TTL)? How to configure an ebgp neighbor (local as 100, neighbor 1.1.1.1, remote–as 20)?
The neighbor is directly connected. The TTL is set to 1.
Describe best practice for configuring BGP peers over parallel links.
Use loopback address instead of physical link address. Can use ebgp–multihop because this command also disables connected checks. Or can just configure disable–connected checks.
Describe iBGP requirements. Configure please.
BGP neighbor in same AS. iBGP assumes peer in same AS and IGP so 64 TTL by default. iBGP neighbors must be full meshed.
Describe the information transfer rules for prefixes received from eBGP peer to iBGP and eBGP peer. (hint: think anti blackhole)
The best prefix can be shared with both eBGP and iBGP peers. This is because the AS path is different and it is not a loop.
Describe the information transfer rules for prefixes received from iBGP peer to iBGP and eBGP peer. (hint: think anti blackhole)
The best prefix can only shared with the eBGP peer. It can be shared with another iBGP peer because it can cause a loop.
Why must iBGP peers be fully meshed?
Because information transfer rules don't allow sharing routes learned from other iBGP peers. So there must be a full mesh of peers. Confederations and router reflectors limit that.
Explain how the next–hop is or is not manipulated with iBGP and eBGP peers.
Between eBGP peers the next–hop is changed to itself for all routes leaving that router. Between iBGP peers the next–hop is not changed unless the router is originating the route.
When an BGP router shares a route learned from an eBGP peer it does not change the next–hop if sharing with an iBGP peer. This can present a next–hop resolution problem for the iBGP peer. How can this be solved. Configure the bgp solution please.
The address must be reachable via the IGP. Using IGP with passive interface, static route, or change the next hop on the ebgp router to itself.
When sending BGP updates what interface will the BGP router use? What if the remote router is anticipating updates coming from a specific interface?
It will use the the interface closest to the destination. Using update source command will configure the desired interface.
Describe the route update process when neighbors first come up and for the duration of the neighborship.
When neighbors first establish they share there entire table – the best BGP router. Then after BGP provides partial, incremental updates based on network changes only.
What is the best way to maximize BGP peer configuration when policy is the same (route–map, as etc)? Configure please.
BGP peer groups
What does this enable?Router bgp 100 bgp listen limit [limit–neighbor–amount] bgp listen range [network]/[prefix] peer–group [peer–group–name] neighbor test peer–group
Dynamic BGP neighbors
What is 4 byte AS? How is it displayed? What's the range? How do neighbors determine it's use?
Provides additional ASN. 0.0 – 65535.65535. Parameters negotiated during startup as optional capability.
What happens if a 2 byte ASN must peer with a 4 byte ASN router? How will AS path look in the 2 byte device?
The 2 byte device must peer with ASN 23456. Any prefixes through 4 byte ASNs will show 23456 prefix.
What are the BGP neighbor states?Describe each state.
ICA OOEIdle, Connect, Active, OpenSent, OpenConfirm, and Established
What are the BGP message types?
OUNKOpen Message, Update Message, Notification Message, and BGP Keepalive
Describe the BGP Open Message.
Open message sent of TCP session established. Used to verify peer and AS configuration. Includes BGP Version, My AS, Hold Time, BGP ID, Optional Parameters
What two values are negotiated using the open message?
BGP Version which defualts to 4 and the Hold Time. Default is 180 but it will use the lowest between the two.
What are some of the options or capabilities exchanged in the Open Message? What happens if a capabilty doesn't match?
Route Refresh, Multiprotocol, Add Paths, Outbound Route Filtering, Graceful Restat, or 4 byte ASN.
What two things are in the Update Message?
Withdrawn routes and NLRI that all have the same Path Attributes
Describe the BGP Notification and keepalive message.
Notification used to convery errors and sent usually during session establishment. Keepalive used for peer liveliness.
Title Attributes
Title Attributes
What are the four BGP PA categories?
– well–known mandatory– well–known discretionary– optional transitive– optional non–transitive
what is well–known mandatory and well–known discretionary?
well known means must be recognized by all. mandatory means must be in every update. discretionary means it doesn't have to be in every update.
What are the well–known mandatory? well–know discretionary?
wkm – next_hop, as–path, originwkd – local_pref, atomic_aggregate
what is optional transitive and optional non–transitive?
Optional – maybe knowntransitive – will forward to between ASnon–transitive – will only stay inside AS
what are optional transitive and optional non–transitive?
OT – Aggregator, CommunityONT – MED, Orginator_ID, Cluster_ID
WKM – ANO. What is the A? What are the four types?
AS_Path. As NLRI pass through AS's the EBGP speakers prepend there AS numbers to the route as it leaves the AS. It starts with the most recent AS.
WKM – ANO. What is N?
Next_Hop IP address. eBGP peers set next_hop to themselves for routes advertised out of AS. iBGP peers only set next_hop to themselves when routes are advertised by themself.
WKM – ANO. What is O?
Origin. It specifies the origin of the route. When BGP has multiple prefixes it uses origin to determine the preferred prefix. IGP (network statement), EGP (never used), Incomplete (redistribution statement)
WKD – LAa. What is L?
L is Local Preference. Local preference is a only shared within an AS. Default local preference is 100. Highest wins.
WKD – LAa. What is Aa?
Atomic Aggregator is an attribute used to tell BGP routers that a route has been aggregated previously. It also signals to the others routers not to make the router more specific and it must keep the atomic_aggregate attribute attached.
OT – AC. What is A?
Aggregator can be sent along with the atomic_aggregate attribute to include the router id and AS of the aggregating router.
OT – AC. What is C?
Community this was originally a Cisco attribute used to make policy enforcement easier. Routers could simply apply a community number to a set of routes and add MED or local pref for the community.
How many bytes in community? How is it displayed?
4 bytes Denoted as Decimal 0 - 42944967296 or AA:NNip bgp–community new–format
What are the four well known communities?
INeNaLInternet, No_export, No_advertise, Local ASInternet means to advertise freelyNo export to ebgp peers (remember E) but can to other confederationsNo advertise (don't advertise to anyone)Local_AS – real name no_export_subconfed tells true meaning. no export to ebgp or outside of sub AS.
Will routers send community by default? Configure router to set community?
No, must be configured to so. router bgp 100neighbor 1.1.1.1 send-community [standard | extended | both ]Extended are for MPLS VPN RT. Neighbor xxx route-map comm out!Route-map comm permit 10 set community 1
After a community has been set will router append new communities like AS?
No, a router will overwrite if configured with a new community. Must used additive keyword Neighbor xxx route-map comm out!Route-map comm permit 10 set community 1 20 additive
What special configuration is needed to match communities?
Must use an ip community list. Standard can match on a number of communities. Extended allows for regex.ip community-list 1 permit xx!Route-map comm permit 10 match community 1 set community none
ONT – MOC. What is M?
Multi Exit Descriminator. MED is a metric which influences entry into an AS. Like metric lowest AS wins.
ONT – MOC. What is OC?
Originator ID and Cluster ID. The Originator ID is the IP address of the of the first Route reflector. The cluster ID is the list of route reflectors the router has passed through. If a router sees it's router id in either list it will drop the route.
BGP Decision Process NSA1WLLAOMNIWhat are the four prereq's the must be meet before routers enter in best path selection?
N is next–hop (WKM) S is synchronization.AS Path (WKM)Neighbor AS present 1st
How is the BGP next-hop derived? What is the default behavior? And how can it be change?
BGP next-hop is derived from the BGP update-source. When not specified it is the interface closest to the destination. Between EBGP peers and iBGP routes locally originated and are directly connected the next-hop self command causes routes to be updated with the update source. With iBGP if the routes are not directly connected it will copy the true next-hop from the RIB. To change use the update source command or a route-map with new next-hop address.
What is BGP syncronization?
Feature is disabled by default. Route must be synchronized before it can be considered as a feasible route. Which means IGP must also now about network. This is only relevant in transit networks when BGP is not configured on the transit routers. They need to know about possible transit networks. To eliminate this probelm syncronization was once enabled. Know the solution is to enable BGP on all transit routers or it is safe to disable when not a transit site.router bgpsynchronization
When is it safe to disable BGP synchronization?
When the AS is not a transit AS or when the AS is fully meshed with IGP. Synchronization really matters when an AS is a transit AS. If the IGP routers between the routers with EBGP neighbors do not know about the external routes then it will create a black hole for traffic. Full mesh ensures every router knows of the route via BGP and not IGP. Then synchronization which by default will not let the routers enter the IGP can be turned off.
Descibe AS Path and Neighbor AS present 1st. Disable both.
AS Path loop prevention happens here. My AS path must not be present. And my neighbors AS must be first in the list.bgp allowas-inbgp enforce-first-as
NSA1WLLAOMNIWhat is W? How to configure?
W is weight. This is a cisco proprietary feature. Highest weight wins. This is specific to one router. There are two functions. 1. For locally originated routes it defaults to 32768. Always want to prefer locally generate BGP routers over ones learned else where. All others 0. 2. When you want to make a local decision about BGP routes learned from someone else you can change the weight.
NSA1WLLAOMNIWhat is L?
Local Pref (WKD). This impacts decision for the entire AS to include an entrire confederation. The default is 100.
NSA1WLLAOMNIWhat is L?
Locally originated. Prefer the route that was learned locally. I, ?, aggregate. Seldom used and weight renders practically useless.
NSA1WLLAOMNIWhat is A? Configure to make less desirable.
AS Path (WKM). Prefer the route with the shortest AS Path. Without any policy configuration this will determine best paths. Can be used to influence inbound and outbound selection. For inbound to my AS I must conifugre an outbound policy in the router with least preferred path prepending more AS. For outbound policy I must configure inbound in my AS on the router with the least preferred path.
What AS_SET with 3 AS how many AS paths does this count for? What aboue AS_CONFED_SEQ and AS_CONFED_SET?
AS_SET is an aggregate set of AS and only counts for one. CONFEDs don't count at all.
NSA1WLLAOMNIWhat is O?
Origin Code (WKM). Prefer route with lowest origin code. I (network statement), E (egp), ? (redistribution)
NSA1WLLAOMNIWhat is M? Configure.
Multi Exit Disc (ONT) influences entry into an AS. The AS numbers must be the same. Lowest wins. When route is brought into BGP it uses 0 for directly connected or copies metric from IGP route. MED stays when inside AS. The MED will only be advertised to another AS if the eBGP neighbor is originating the route. AS receiving the MED will not advertise it to another AS. If the eBGP neighbot is not originating the route the MED will be remove or is said to be missing.
Missing MED defaults to what? How to prevent the default from beating a set MED.
Missing MED defaults to 0. To prevent the 0 from beating a set MED configure router bgp 100 bgp bestpath med missing–as–worst

What do the following commands do?


bgp always-compare-med


bgp deterministic-med

MED is usually only compared between when two prefixes are coming from one AS. Always compare allows MED to be considered if even routes are from different AS.



Deterministic MED cause routes to be group together by AS before comparing.

Which route is the oldest? When it's time to compare which route is compared first? Which route wins? What happens when bgp always-compare-med is enabled? What happens when bgp deterministic-med is also added.




entry1: AS(PATH) 500, med 150, external, rid 172.16.13.1


entry2: AS(PATH) 100, med 200, external, rid 1.1.1.1


entry3: AS(PATH) 500, med 100, internal, rid 172.16.8.4

The oldest route is entry 3 at the bottom.




Start with Entry 1 vs Entry 2.




Entry 2 beats 1 because of RID; MED cannot be compared. Entry 2 beats 3 because external over internal; MED cannot be compared.




Entry 1 beats 2 because of MED. Entry 3 beats 1 because of MED.




Entry 1 and 3 are grouped together and compared. Entry 3 wins. Entry 3 beats entry 2.





NSA1WLLAOMNIWhat is N?
Neighbor Type. Prefer ebgp over ibgp.
NSA1WLLAOMNIWhat is I?
IGP metric to next–hop.
What if all routes are still equal after the 8 step selection process (RN)? Configure.
Oldest Route/Router–ID and Neighbor IP address. If maximum–paths is enabled and the route same neighboring AS (for ebgp) or same next–hop (for igbp) then install equal–cost routes in the RIB. If it is not enabled prefer the route with the oldest route/EBGP or lowest BGP router ID. Lowest neighbor address.
What methods can you use to influence inbound routing?
AS Path prepend, MED, and community attribute coupled with local-preference (RFC 1998)
What methods can you use to influence outbound routing?
Weight and local preference
Title Prefix Generation
Title Prefix Generation
What are ways to originate NLRI?
Network Statement, Redistribution, Aggregation, Conditional Route Injection
How does network command with BGP differ from IGP?
The network command with BGP adds prefix to BGP if the network is in the route table. Unlike IGP the network does not have to be a directly connected interface. And unlike IGP the network command does not enable dynamic neighbor discovery using a wildcard discovery.
Two common ways to add prefixes to BGP.Configure please
Redistribute and network statement
Is Auto-summary by default?
Not any longer
When you show ip bgp there are three IP addresses listed. (65001 64955 65003) 65089 172.16.254.226 (metric 20645) from 172.16.216.252 (172.16.216.252)
The first is the BGP next–hop address. The second is the BGP peer IP address. The third is the BGP router–id.
command: show bgp prefixes that are in the routing tablecommand:show detailed information about bgp route. Route can appear here and not in the route table.command: summary of bgp routing table
show ip route bgp x.x.x.xshow ip bgp x.x.x.xshow ip bgp summary
command: show advertised routes to a particular neighborcommand:to see received routes from a particular neighbor
show ip bgp neighbor x.x.x.x advertised–routesshow ip bgp neighbor x.x.x.x routes
Title ORF, Soft cfg refresh
Title ORF, Soft cfg refresh
What features enables BGP router to tell what prefix it wants it's neighbor to send? Where can it be configured?
Outbound Route Filtering (ORF). Between eBGP neighbors only.
What is the command to enable ORF?
router bgp 100 neighbor 1.1.1.1 capabilities orf prefix-list [send|receive|both]must configure send on the router you want to send the list from and recieve on the router who is receiving the list.
What does in direction do on prefix-list on ORF router?
The in direction tells opposite router this is what I want to come into me. Inbound prefix becomes outbound on the opposite side.
What command to see the received prefix-list?
show ip bgp neighbor x.x.x.x received prefix-list

What are the ORF components included int the Route-Refresh Message and documented in the RFC?

AFI/SAFI - e.g. IPv4/Unicast


ORF Type - Cisco value is 128


Action - Add or Remove prefix


Match - Permit or Deny prefix

What does BGP route refresh do? How to configure.
Provides the ability for router to store BGP routes locally in a table. With the execution of clear ip bgp * soft in the router will apply policies to these routes.router bgp 1 neighbor 1.1.1.1 soft-reconfiguration inbound
What does clear ip bgp * soft in do when route refresh is not configured?
It will ask neighbor to send new route to me.
What does clear ip bgp * soft out do when route refresh is not configured?
The behavior doesn't change with route refresh. The device will send new routes to peers.
Title Conditional Advertisement
Title Conditional Advertisement
Describe Conditional Exist Map Advertise Map
Using a route-map tied to a prefix list you can tell router to adverise the prefixes identified in the advertise map if the routes exist in routing table that match exist-map.
Describe Conditional Non-Exist Advertise Map
Using a route-map tied to a prefix list you can tell router to adverise the prefixes identified in the advertise map if the routes do not exist in routing table that match exist-map.
Describe Conditional Inject Map Exist Map
Using two route-map; one tied to a prefix (summary) you want to advertise (inject map) and the other tied to two prefixes (exist) will tell router to inject a subset of a summary route. The exist map will have route-map that matches the summary route and match ip route-source.
Title Convergence, PIC, Add Paths
Title Convergence, PIC, Add Paths
BGP convergence is slow. What default helps speed EBGP convergence? How does it help?
router bgp 100 bgp fast–external–falloverWhen the directly connected link with the EBGP neighbor on it goes down it brings down the EBGP neighbor. That will also remove the routes.
What helps speed iBGP and eBGP mulithop convergence by taking neighbor down?
Fast Fallover Session Deactivation. Is like Fast–External–Fallover in that it goal is to bring down the neighbor and all of it's associated routes. It is configured per neighbor and watches the IGP route to that neighbor.router bgp 200 neighbor 155.1.58.8 fallover
What default feature helps speed iBGP and eBGP mulithop convergence by removing specific routes?
Next Hop Tracking. It will watch the next–hops for BGP routes. If there is no longer a route to next–hop the route is removed. There is a 5 sec delay to allow IGP to converge. bgp nexthop trigger enable bgp nexthop trigger delay 5
BGP only shares best path with neighbors. What feature allows for additional paths to be shared? Is any negotiation required for this feature? What is added to the prefix?
BGP Additional Paths. It is negotiated in the open message. Path IDs are added similiar to VPN RD. It only works with iBGP peers.
What are the three general steps to configure Add Paths?
First configure how to send or receive additional paths capability. Second configure to send how many best paths. Third advertise additional paths to neighbors. Router bgp 100 bgp additional-paths send receive bgp additional-paths select group-best all neighbor 192.168.2.2 advertise additional-paths best 3
What keywords are in the show ip bgp which indicate the route is an additional path?
With show ip bgp you will see an "a" to the left of the prefix. WIth show ip bgp x.x.x.x you will see best or best 2 or best 3. The all keyword will show if you are using the all as selection criteria.
What is feature is required to install both prefixes? What caveat must be remembered?
Prefix Independent Conversion or PIC. The next hop must be different for the routes to be installed?router bgp 1bgp additional-paths install
Title RR and Confederations
Title RR and Confederations
Why full mesh requirement? What are two other ways to resolve the full mesh requirement?
1a. To prevent BGP routing loops within AS IBGP routers are only allowed to share routes one route deep.1b. Full mesh is required for all routers that are potential transit routers for ebgp learned routes (example OSPF area 0 only routers need to be ibgp full meshed). They need to know about all external routes. If the two routers with ebgp connections could share the routes with the IGP then this would not be the case. The IGP however cannot support that many routes so the routes must be shared via IBGP and full mesh because of loop prevention techniques.Full mesh within iBGP because rules only allow to share ebgp routes and locally learned routes with iBGP. the AS number eBGP routes are different and locally learned are new routes so now loops can be formed.2. Router reflectors and confederations.
What are route–reflectors? What are the reflection rules? Configure please.
They break rules and allow routes to be share with other internal neighbors.– They reflect routes from non–client peers to peers but to not to other non–client ibgp peers.– reflect routes from RR peers to non–client RR peers and other client RRs peers.
What does RR use for loop avoidance? What does each mean?
Originator and Cluster ID attributes [ONT]Cluster list keep tracks of the RR. If a RR sees it's ID in the cluster it will drop the advertisement. Originator ID is the BGP speaker that originated the route. If router sees it's ID it will drop the route.
What's the use of cluster when you have router-id?
Router ID will work if that router see's the route again. What about Redundant Route reflectors? They will be in the same cluster with the same cluster id. You don't want them sharing routes between each other.
How to configure cluster–id?
bgp router–idbgp cluster–id
What are confederations?
Creates sub–AS inside of AS. Still must configure a full mesh inside of AS.
Which as number do you use under router–bgp
The sub–asrouter bgp [sub-as]
when configuring peers inside the sub–as which remote–as number do you use?
the sub–asrouter bgp neighbor x.x.x.x remote–as
what additional command is needed when configuring a neighbor inside another sub–as
router bgp 65001neighbor x.x.x.x remote–as 65002bgp confederation peer 65002
true ebgp peers will not know about the sub as. How do you configure the real as number
router bgp 65001bgp confederation identifier 200
What MED option is availble for confederations?
MED must be enabled. Missing MED worst.bgp bestpath med confedbgp bestpath med confed missing-as-worst
Title Aggregation, as-set
Title Aggregation, as-set
What are the two ways to create summaries with bgp?
1. Create static route to null 0 for network and then use network command in bgp. 2. aggregate–address . In both cases the aggregate will only be created if the route exists in network.
Where can you summarize? Describe what happens to a route when it it summarized.
1. Can summarize between iBGP peers and eBGP peers. 2. AS–Path info is lost.
Describe the summary–only option for bgp aggregate address.
summary–only suppresses the more specific routes and only advertises the summary route. The routes that are suppressed will have a s" in front of it.
What is the purpose of the as–set command with BGP summarization? It also cause route to inherent community attribute.
The purpose of the as–set command is to prevent loops. The AS path information is lost whenever BGP aggregation is performed. The AS–set is an unordered list of paths. However when the prefixes all have the same path information the aggregated route looks like ordered list (AS–SEQ). When it is not then the path shows up as (AS–SET) with parenthesis.
Describe the suppress–map option for bgp aggregate address.
The suppress–map points to a route–map which specifies a subset of networks to be suppressed. The networks must be permit by the ACL or prefix–list and route–map.
Describe the unsuppress–map option for bgp aggregate address.
The unsuppress–map points to a route–map which specifies a subset of networks to be unsuppressed. The networks must be permit by the ACL or prefix–list and route–map.
What is the attribute map? What attributes can be changed?
This supports changes to attributes on the aggregate route. Origin, community, etc.
Title Local AS, allow AS in, remove private AS
Title Local AS, allow AS in, remove private AS
What does the neighbor x.x.x.x local-as x do?
This allows a remote peer to connect to me using a different AS from the one configured in the router bgp x.
What does the neighbor x.x.x.x local-as x no-prepend do?
Default (different( behavior is for this router to prepend the local AS to inbound updates. It will have the neighbor eBGP AS plus the local AS. The no prepend removes the local AS
What does the neighbor x.x.x.x local-as x no-prepend replace-as do?
Default behavior is to prepend both local and router bgp as to outbound packets. This allows the router bgp as to be removed.
What does the neighbor x.x.x.x local-as x no-prepend replace-as dual-as do?
Dual AS allows the remove eBGP peer to connect to either local AS or the AS int the router bgp x. If the peer connects to the router bgp AS the local AS will be removed instead of the router bgp on outbound updates because of the replace-as command.
What does allowas-in do? When would you want to use this?
Tells router it can allow it's own AS in the list. Usually with L3VPNs when customer peer via BGP. They will see there own ASN.
What if the allowas-in cannot be configured?
If you cannot get to the CE router another option is as-override. This tells the PE to drop last AS off and replace with it's SP AS number.



R1(AS1) ---R2 (PE - AS2) ---R3 (PE - AS2) --- R4 (AS1)




R3 with as-override configured outbound would replace AS1 with AS2.

What command allows SP to operate with private AS for their customers? Where is this applied?
remove-private-as. It is not applied in the customer network. It is applied on all SP routers with upstream EBGP peers. And this must be the first AS in path. If another AS is in front of the private it won't be able to remove it.
Title AS Path Filter and Regex
Title AS Path Filter and Regex
What are the methods to filter?
1. applied to neighbor – distribute–list with standard or extended ACL, prefix–list, filter list (as–path ACL) 2. aggregate with summary–only, suppress–map3. route–map which supports community, ACL, and prefix
Describe as–path filtering?
It is applied directly to neighbor and is used match on AS Path.
As it relates to as–path filtering what is regular expressions?
regex for short. They are used to match the as path. There are several characters.
What does ip as–path access–list permit 850 do?
It will permit any route where 850 exists in the AS path. It can be 3850 or just 850.
^ and $ are common regex. What are they?
^ matches the beginning of the line and $ matches the end of the line. permit ^5610_148_286$ means 5610 must be at beggining and 286 must be at end. permit _5610_148_286_ means they can be anywhere in the AS path.
How to match an empty AS with regex?
permit ^$
How configure match all with regex?
permit .* The dot means any character and the * means zero or more occurrences of that character.
How to match path 5610, 14, 284 with regex?
permit ^5610_148_286$
How to test regex?
show ip bgp regexp
Where to apply BGP distribute list to filter routes? Configure please.
It is applied to the neighbor or peer group. Inbound and outbound direction. Apply a standard are extended access–list.
When using an acl for filter list what does ip access–list 1 permit 172.16.32.0" match? What if you only wanted to match 172.16.32.0/24"
It matches any subnet with 172.16.32.0. It has an implicit 0.0.0.0. The extended acl would need to specify subnet using the host command. ip access–list 100 permit ip 172.16.32.0 host 255.255.255.0.
Title Prefix List and Backdoor
Title Prefix List and Backdoor
Where to apply BGP prefix–list to filter routes? Configure please.
It is applied to neighbor or peer group. Inbound or outbound direction.
Explain how prefix lists work. ip prefix–list permit 172.16.32.0/22. ge 20 le 23. What does the /22 mean?
The /22 specifies the possible matched networks. 172.16.32.0 with a /22 means 172.16.32.0 – 172.16.35.255. 111111100.
Explain how prefix lists work. ip prefix–list permit 172.16.32.0/22. ge 20 le 23. What does the ge 20 and le 23 mean?
The ge means greater than or equal to. The le means less than or equal to. Any networks between 172.16.32.0 – 172.16.35.255 whose mask is ge 20 or le 23 will be matched.
How to match default with prefix–list.
0.0.0.0/0ip prefix–list demo permit 0.0.0.0/0
How to configure match all with prefix–list.
0.0.0.0/0 le 32
How to use route–maps to filter routes?
Access–list, prefix–list, or as–path can be used in a route–map.
Why would I use the route–maps to filter routes instead of the distribute–lists, prefix–lists, or filter–list (as–path). Two reasons.
1. Because route–maps can be named making administration and operation slightly easier.2. Route maps can also be used to change attributes.
When two ASs have a backdoor link with IGP routing and also eBGP links to public AS, how to configure so that the networks learned over backdoor links are preferred. By default the eBGP routes will have 20 and IGP will be higher.
network x.x.x.x backdoor. This will cause route to be locally generated making it the best BGP route and changing the AD to 200. The backdoor says do not advertise to any other neighbor.