Private AS Numbers:

When you look at router bgp ? you see (1-65535), which the range of 64496-65535 are actually reserved or Private AS’s, which should not be advertised Externally as Private IP’s on your LAN should not be advertised to External networks.

Also to note, you cannot use AS 0, and will get an “Invalid input” error if you try to issue “router bgp 0” on a router.

***Also some Cisco docs use the acronym ASN to indicate AS Number and some not, so be prepared if you see ASN on your exam and understand it is referring to AS Number.***

BGP RID:

As it wasn’t specifically covered (aside from demonstrating manually setting it), the RID has not come into play much, except as a tie-breaker for Best Path selection quite a few times in labs with all things equal – “bgp router-id x.x.x.x” is the command once more.

One thing to note with manual RID configuration, if you do it with live Adjacencies, it will drop those Adjacencies but they will quickly reform using the new RID configured.

Now without manually setting it, it derives its RID the same way OSPF or EIGRP does, which is first the highest logical interface IPv4 address, follow by the highest Physical interface IPv4 address.

To see the rid you can either do “sh ip bgp” / “sh ip bgp x.x.x.x”, however if you have no advertised routes you can also use “sh ip bgp summ” :

R5#sh ip bgp summ
BGP router identifier 172.16.11.1, local AS number 5
BGP table version is 7, main routing table version 7
6 network entries using 720 bytes of memory
6 path entries using 312 bytes of memory
1/1 BGP path/bestpath attribute entries using 124 bytes of memory
0 BGP route-map cache entries using 0 bytes of memory
0 BGP filter-list cache entries using 0 bytes of memory
BGP using 1156 total bytes of memory
BGP activity 6/0 prefixes, 6/0 paths, scan interval 60 secs

Neighbor        V           AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
172.12.15.1     4          123      97     107        7    0    0 01:33:03        0
R5#

R5’s highest logical interface, so remember there is a couple ways to get that RID if commands are disabled on exam day, in a squeeze you could go to a neighbor if you have access to its console and do “sh ip bgp nei” to get the RID of the remote Neighbor.

Another important note – If two BGP neighbors are configured with the same RID will not form an Adjacency, and the console will continuously spit out an error saying “(BGP identifer wrong)” – So be sure to note that as well!

Route Redistribution between IGP and EGP:

Although with IGP’s route redistribution generally goes both ways (but doesn’t have to), you may need to occasionally redistribute IGP routes into BGP, and there are three ways to get that to happen:

  • The network command
  • Static route redistribution
  • Redistribution of routes discovered by an IGP

The third of this list is strongly recommended by Cisco to avoid when possible as it can lead to routing loops fairly easily. Using the ‘network’ command is almost always the best way to go about doing it.

Redistributing route from BGP into an IGP is almost never a good idea, or going to be required for a task, and doing such can cause a lot of issues (considering BGP can have thousands of routes).

Bottom line, NEVER redistribute BGP into an IGP unless you have a very good reason!

AND THAT IS IT FOR BGP! Whew. Up next is a look at CEF, but definitely not tonight! 🙂