ip addresses in same subnet being learned differenly

Unanswered Question
Mar 2nd, 2010

trying to reach ip addresses from our data center to our remote office

we are running ospf internally

the remote office sites have 2 routers..with 172.20.255.2 and 172.20.255.3 configured as a subinterface-glbp address is 172.20.255.1

we have a host device-172.20.255.4 all of these are configured with mask 255.255.255.248

for ex

interface GigabitEthernet0/0.1720
encapsulation dot1Q 1720
ip address 172.20.255.3 255.255.255.248
no snmp trap link-status
glbp 4 ip 172.20.255.1
glbp 4 weighting track 1 decrement 100
end

Yet I am unable to reach the router interfaces or the glbp vip address but can ping the .4 host address

I look at routing and notice this

/* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;}

DATACENTER1#sh ip ro 172.20.255.4

Routing entry for 172.20.255.0/29

  Known via "ospf 711", distance 110, metric 1

  Tag 13979, type extern 2, forward metric 1

  Last update from 10.10.1.2 on Vlan101, 2w3d ago

  Routing Descriptor Blocks:

  * 10.10.1.3, from 10.1.1.10, 2w3d ago, via Vlan101

      Route metric is 1, traffic share count is 1

      Route tag 65000

    10.10.1.2, from 10.10.1.2, 2w3d ago, via Vlan101

      Route metric is 1, traffic share count is 1

      Route tag 13979

/* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;}

DATACENTER1#sh ip ro 172.20.255.1

Routing entry for 172.20.255.0/30

  Known via "static", distance 1, metric 0

  Redistributing via ospf 711

  Advertised by ospf 711 subnets

  Routing Descriptor Blocks:

  * 10.11.24.10

      Route metric is 0, traffic share count is 1

sh ip rout static

/* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin:0in; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-fareast-font-family:"Times New Roman"; mso-fareast-theme-font:minor-fareast; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;}

ip route 172.20.255.0 255.255.255.252 10.11.24.10

My question is /30 vs./29 which is configured on the host devices and gw

how would this affect pinging one device instead of another on the same subnet in the same office?

I have this problem too.
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Jon Marshall Tue, 03/02/2010 - 09:22

172.20.255.0 /30  =  172.20.255.1 -> 2 with .3 being the broadcast address. So that covers the physical and virtual addresses on your routers hence the reason you can't ping them.

172.20.255.0/30 does not include 172.20.255.4 so the /29 route is used when you try to ping that address and is therefore routed correctly. You need to work out why you have this static and why the same addressing has been used in the remote site.

Jon

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