Cisco and the AD of BGP

Hi all,

I was wondering about the AD of BGP and his implementation on various vendors.

For Alcatel, Juniper and Huawei, they use the same value for i and e BGP. And as you know, Cisco use two : 20 and 200.

On Cisco Routers, the Path Selection is based on the 7th* Attribute (eBGP > iBGP) ; whatever is the AD of eBGP/iBGP.


  • For Exp., if AD of eBGP = 50 and iBGP = 30. The router chose the eBGP route.

In must cases, iBGP is not choosed because of his high AD.

Can some one explain me please, why this "separation" between iBGP and eBGP by Cisco ?

Thanks in advance,

Brgds,

Fay ONIS

(*) http://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html

Comments

  • The AD has nothing to do with BGP path selection based on route type ebgp/ibgp. this has as result bgp routes in the loc-RIB (BGP Table).

    AD is considered when choosing between protocols to install a route in the routing table.

    I believe AD 200 is used for iBGP to avoid considering it instead of IGP, otherwise it can create loops. a router has routes to the same destination inside the AS via iBGP and an IGP. If the iBGP has a small AD then it prefers to route to this destination via iBGP. For DV protocols this has bad consequences since it will cause intermittent reachability (route removed from RT, not advertised anymore etc). So it is basically loop prevention: for destinations inside the AS, IGP should be preferred. 

    Does this answer your question?

  • Hi
    Mihai,

    Thanks for the reply. 

    Yes,
    i'm ok with these ideas.

    Simply,
    Why to not make it "simple" like other vendors, using eBGP for
    external routes and IGP for Internal routes.

     

    From
    my understanding, iBGP is used when there is no IGP. Adding to that,
    historically EGP is designed for exchanging routes between AS (or Entreprises).

    Brgds,

    Fay ONIS

  • peetypeety ✭✭✭

    Go read a previous post I've made on this very topic: http://ieoc.com/forums/p/29338/240094.aspx#240094

    Essentially, the PSA is used within BGP for BGP to select a path (kinda obvious, but worth writing explicitly for the sake of explanation). As the best routes are presented to the master RIB, AD is used to pick the protocol from which it's going to pick a particular path for a routing entry.

  • Hi gentelmen,

    Thanks for all your reply. [:)]

    Brgds,

    Fay ONIS

  • ya sure !

    At INE, Inc. 8745 Technology Way. Suite B. Reno, NV 89521.

    Or, Cisco Systems, Inc. 170 West Tasman Drive. San Jose, California 95134-1706

    [:D][:D][:D]

    Fay ONIS

Sign In or Register to comment.