Cisco Doc - Config BGP Loopback

8/11/2019 Cisco Doc - Config BGP Loopback

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Cisco Doc: Config iBGP/eBGP with/without Loopback @

Introduction on BGP

Exterior Gateway protocol (EGP)perform inter-domain routing in TCP/IP network

Establish TCP connectivity (TCP port # 179)with other BGP router peer in order to exchange BGP updates

eBGP session2 BGP router peers have different AS #

iBGP session2 BGP router peers have same AS#

Default Peer relationship is established using IP@ of the interface closest to the peer.

Any operational interface (inc loopback int) can establish peer relationship using neigbour update-source

command.

iBGP Configuration without Loopback

iBGP Configuration with Loopback

Router R1-AGS

interface Serial1

ip address 10.10.10.1 255.255.255.0

!

router bgp 400

neighbor 10.10.10.2 remote-as 400

end

Router R6-2500

interface Serial0

ip address 10.10.10.2 255.255.255.0

!

router bgp 400


end

Both routers have same AS#: AS 400

R1-AGS S1 int IP@: 10.10.10.1/24

R6-2500 S0 int IP@: 10.10.10.2/24

Router R1-AGSinterface Serial1

ip address 10.10.10.1 255.255.255.0

!

Interface loopback1

Ip address 1.1.1.1 255.255.255.255

!

router bgp 400

neighbour 2.2.2.2 remote-as 400

neighbour 2.2.2.2 update-source loopback2

!

Ip route 2.2.2.2 255.255.255.255 10.10.10.2

end

Router R6-2500

interface Serial0

ip address 10.10.10.2 255.255.255.0

!

Int loopback2

Ip address 2.2.2.2 255.255.255.255

!

router bgp 400



!

Ip route 1.1.1.1 255.255.255.255 10.10.10.1

end


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eBGP Configuration without Loopback (Different AS#)

eBGP Configuration with Loopback Interface

Router R1-AGS

interface Serial1

ip address 10.10.10.1 255.255.255.0

!

router bgp 300


end

Router R6-2500

interface Serial0

ip address 10.10.10.2 255.255.255.0

!

router bgp 400


end

AS 300AS 400

Router R1-AGS

interface Serial1

ip address 10.10.10.1 255.255.255.0

!

Int loopback1

Ip address 1.1.1.1. 255.255.255.255

!

router bgp 300


neighbour 2.2.2.2 ebgp-multihop 2!--- This command changes the ttl value in!--- order to allow the packet to reach the

!--- external BGP peer which is not directly

!--- connected or is using an interface other

!--- than the directly connected interface.


!

Ip route 2.2.2.2 255.255.255.255 10.10.10.2

end

Router R6-2500

interface Serial0ip address 10.10.10.2 255.255.255.0

!

Int loopback2

Ip address 2.2.2.2 255.255.255.255

!

router bgp 400


neighbour 1.1.1.1 ebgp-multihop 2


!

End

Ip route 1.1.1.1 255.255.255.255 10.10.10.1


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Verify BGP connection with peers: show ip bgp neighbours/summary

Verify iBGP:Without Loopback:R1-AGS# show ip bgp neighbors | include BGP

BGP neighbor is 10.10.10.2, remote AS 400, internal link

BGP version 4, remote router ID 2.2.2.2

BGP state = Established, up for 00:04:20

BGP table version 1, neighbor version 1

R1-AGS#

R1-AGS(9)# show ip bgp summary

BGP router identifier 10.1.1.2, local AS number 400

BGP table version is 1, main routing table version 1

Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd

10.10.10.2 4 400 3 3 1 0 0 00:00:26 0

With Loopback:R1-AGS# show ip bgp neighbors | include BGP

BGP neighbor is2.2.2.2, remote AS 300, internal link




R1-AGS#




2.2.2.2 4 400 3 3 1 0 0 00:00:26 0

Verify BGP:Without Loopback:R1-AGS# show ip bgp neighbors | include BGP

BGP neighbor is 10.10.10.2, remote AS 400, external link








10.10.10.2 4 400 3 3 1 0 0 00:00:26 0

With Loopback:R1-AGS# show ip bgp neighbors | include BGP

BGP neighbor is 2.2.2.2, remote AS 400, external link




External BGP neighbor may be up to 2 hops away.





2.2.2.2 4 400 3 3 1 0 0 00:00:26 0


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Cisco DocumentBGP Best Path Selection Algorithm

The algorithm is used to decide best routing path to the destination and avoid route redundancy.

Why Router ignore the path:

Path is not sync.

Path for which the NEXT_HOP is inaccessible

Path from the eBGP peer where the local AS# appears in the AS_PATH

The update that does not contain the NB AS# as the 1stAS# in the AS_SEQ Path that mark as receive-only (show ip bgp route)

How the Best Path Algorithm works?

Prefer the path that has higher WEIGHT/LOCAL_PREF

Prefer the path with shortest AS_PATH

Prefer the path with lowest Origin type (IGP


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Load-Balancing in Cisco

2 types: PerDestination and Per-Packet Load Balancing

Per-destination: The router distributes the packets based on the destination address. All packets for Dest_1 1st

path,

all packets for Dest_2 go over 2nd

path. Router need to build a route-cache entry for every destination address,

instead of every destination network, as is the case when only a single path exists. Therefore traffic for different

hosts on the same destination network use different paths.

Advantage: Preserves packet order.

Disadvantage: May cause unequal usage of the links. If one host receives the majority of the traffic all packets useone link, which leaves bandwidth on other links unused. Core backbone routers consume large computer resource to

perform the load balancing (Maintain the cache to route traffic to destination hosts.)

Per-packet load-balancing: The router distributes the packet towards 1 destination hosts to several links equally.

Advantage: Guarantees equal load across all links.

Disadvantage: Packets may arrive out of order at the destination because differential delay may exist within the

network. Extensive processing load is created at router to look for less utilized interface packet forwarding from

route table. Not suitable for high speed interface/transmission as delay created from the mechanism

aforementioned.

**For IOS, per packet load balancing will disable the forwarding acceleration by a route cache. This is because theroute cache information includes the outgoing interface. In order for per-packet load balancing to achieve equal link

utilizations, the forwarding process determines the outgoing interface for each packet by looking up the route table

and picking the least used interface. but is a processor intensive task and impacts the overall forwarding

performance. This form of per-packet load balancing is not well suited for higher speed interfaces.

By default, most Cisco routers are configured with fast switching enabled under interfaces. This is a demand caching

scheme that does per-destination load-balancing.

To set per-packet load-balancing, enable process switching by disable fast switching:

Router# config t

Router(config)# interface Ethernet 0

Router(config-if)# no ip route-cacheRouter(config-if)# Z

Now the router CPU looks at every single packet and load balances on the number of routes in the routing table for

the destination. This can crash a low-end router because the CPU must do all the processing . To re-enable fast

switching, use these commands:

Router# config t

Router(config)# interface Ethernet 0

Router(config-if)# ip route-cache

Router(config-if)# Z

CEF: Cisco Express Forwardingdoes Load-balancing faster

http://www.cisco.com/en/US/products/hw/modules/ps2033/prod_technical_reference09186a00800afeb7.html

http://www.cisco.com/c/en/us/support/docs/ip/express-forwarding-cef/18285-loadbal-cef.html

Load Balancing

http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13762-40.html

http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipswitch_cef/configuration/xe-3s/isw-cef-xe-3s-book/isw-cef-

load-balancing.pdf
http://www.cisco.com/en/US/products/hw/modules/ps2033/prod_technical_reference09186a00800afeb7.htmlhttp://www.cisco.com/en/US/products/hw/modules/ps2033/prod_technical_reference09186a00800afeb7.htmlhttp://www.cisco.com/c/en/us/support/docs/ip/express-forwarding-cef/18285-loadbal-cef.htmlhttp://www.cisco.com/c/en/us/support/docs/ip/express-forwarding-cef/18285-loadbal-cef.htmlhttp://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13762-40.htmlhttp://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13762-40.htmlhttp://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipswitch_cef/configuration/xe-3s/isw-cef-xe-3s-book/isw-cef-load-balancing.pdfhttp://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipswitch_cef/configuration/xe-3s/isw-cef-xe-3s-book/isw-cef-load-balancing.pdfhttp://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipswitch_cef/configuration/xe-3s/isw-cef-xe-3s-book/isw-cef-load-balancing.pdfhttp://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipswitch_cef/configuration/xe-3s/isw-cef-xe-3s-book/isw-cef-load-balancing.pdfhttp://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipswitch_cef/configuration/xe-3s/isw-cef-xe-3s-book/isw-cef-load-balancing.pdfhttp://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13762-40.htmlhttp://www.cisco.com/c/en/us/support/docs/ip/express-forwarding-cef/18285-loadbal-cef.htmlhttp://www.cisco.com/en/US/products/hw/modules/ps2033/prod_technical_reference09186a00800afeb7.html


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Cisco Doc - Config BGP Loopback